The popular car-service app is using a billion-dollar venture-capital war chest to crush the yellow and green competition by slashing prices on its black cars to below the fixed rate of regular taxis.

Uber’s 20 percent discount, which started Monday, means users can get a ride from Williamsburg to the East Village for $15, one dollar less than a cab. A ride from Grand Central to downtown is $22, which is about $2 less.

“We want to be the most affordable ride in town,” said Josh Mohrer, general manager of Uber for New York City.

The lower prices aren’t permanent — and they won’t apply to UberBlack’s luxury cars, which include Mercedes and Cadillacs. The company would not say how long the discount will last.

Uber also has no plans to change its controversial surge pricing during rain storms and holidays.

Uber — which is allowed by law to charge as little as it wishes to for black cars — is able to cut prices thanks to a $1.2 billion investment windfall raised from bigmoney backers last month.

The cuts are part of a nationwide price war Uber hopes to start with traditional cabs. It has e already slashed prices 25 percent in 25 other cities in recent weeks, including Washington, DC, Boston, and Los Angeles.

The company’s move also comes as Lyft, a ride-share app, has been looking to launch in Brooklyn.

Lyft drivers are not professional cabbies, but offer rides in their personal cars for donations.

That system is legal in cities like San Francisco and LA, but currently illegal in New York City.

Lyft is looking at ways to make its business model work in the Big Apple and is recruiting Brooklyn drivers.

Uber’s tactics have gotten a lot of backlash from the taxi industry.

Ethan Gerber, executive director for the medallion-holders group the Greater New York Taxi Association, blasted the move to undercut cabs as “a desperate attempt to penetrate a market in an unsustainable way.”

Other industry groups have also said that yellow cab drivers are safer because they must meet more stringent standards than livery drivers.

But TLC Commissioner Meera Joshi said that as long as Uber’s cars meet the city’s safety and consumer protection standards, riders should be able to pick.

“The consumer can choose which service best serves their needs, whether that’s based on price, vehicle type, base location or something else,” she said.